In my never-to-be-humble opinion, Nicole and EJ have the abillity & willingness to not only accept each others dark side, but they encourage it when the need suits them...and the chemistry simply drips off them without effort. And if Sami isn't willing to accept EJ's dark side, they have no chance unless he completely rejects ALL things DiMera - including the money, the power and the name itself while she has to reject all things Brady - including her family and the support & stability and (somewhat level of) sanity they offer her when she continually jacks up her life so bad.